US20190182921A1

US20190182921A1 - Dimming controllers and dimming methods capable of receiving PWM dimming signal and DC dimming signal

Info

Publication number: US20190182921A1
Application number: US16/199,367
Authority: US
Inventors: Chun Hsin Li; Wei Cheng Su; Hung Ching Lee; Chung-Wei Lin
Original assignee: Leadtrend Technology Corp
Current assignee: Leadtrend Technology Corp
Priority date: 2017-12-13
Filing date: 2018-11-26
Publication date: 2019-06-13
Anticipated expiration: 2038-11-26
Also published as: TW201929600A; US10397997B2; TWI658748B

Abstract

A dimming controller is capable of receiving a dimming signal to dim light-emitting device no matter the dimming signal is of a first type or of a second type. A type identifier identifies whether the dimming signal received from an input node is of the first type or of the second type, to accordingly generate a selection signal. A signal converter generates a first signal in response to the dimming signal, and the first signal is of the first type. A multiplexer has two inputs receiving the first signal and the dimming signal respectively, and, in response to the selection signal, forwards one of the first signal and the dimming signal to a driver driving the light-emitting device.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Taiwan Application Series Number 106143645 filed on Dec. 13, 2017, which is incorporated by reference in its entirety.

BACKGROUND

The present disclosure relates generally to dimming controllers and dimming methods, and, more particularly, to dimming controllers suitable of receiving a dimming signal no matter it is a pulse-width-modulation (PWM) signal or a direct-current (DC) signal.
Light emitting diode (LED), due to its characteristics in high power efficiency, compact product size, and long lifespan, has been widely adapted by lighting appliances and backlight modules. Until recently, most of cold cathode fluorescent lamps (CCFL) in the backlight modules of TV or computer display panels, for example, are replaced by LED modules.
LED modules usually need dimming controllers to perform light dimming, so as to adjust the luminance of a display panel for example. There are two different methods in the art to dim the luminance of a LED module: PWM dimming and DC dimming. PWM dimming, also named digital dimming, employs a PWM or digital signal that jumps quickly back-and-forth between levels of “0” and “1” in logic to determine the duty cycle of a LED module, the ratio of the time when the LED module emits light to the cycle time of the PWM signal. For example, when the PWM signal is “1” in logic, the luminance of the LED module is in its maximum, and when the PWM signal is “0”, it is zero, not emitting light. In other words, PWM dimming makes a LED module blinking. In contrast, DC dimming, also known as analog dimming or resistive dimming, makes a LED module emitting light continuously while the luminance of the LED module corresponds to the voltage level of a DC or analog signal.
For having more market share, a dimming controller should accommodate a dimming signal no matter the dimming signal is of PWM or of DC, and provide appropriate luminance control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. These drawings are not necessarily drawn to scale. Likewise, the relative sizes of elements illustrated by the drawings may differ from the relative sizes depicted.

The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

FIG. 1 illustrates dimming controller 10 that controls the luminance of light-emitting device LT via power transistor MNDRV;

FIG. 2 demonstrates dimming controller 10 a;

FIG. 3 shows the correlation between dimming signal S_DIM, saw-wave signal S_SAWand PWM signal S_PWM;

FIG. 4 exemplifies the waveform of dimming signal S_DIM;

FIG. 5 shows dimming methods 60 a in use of dimming controller 10 a in FIG. 2;

FIG. 6 demonstrates dimming controller 10 b; and

FIG. 7 shows dimming methods 60 b in use of dimming controller 10 b in FIG. 6.

DETAILED DESCRIPTION

According to embodiments of the invention, FIG. 1 illustrates dimming controller 10 that controls the luminance of light-emitting device LT via power transistor MNDRV.
Power transistor MNDRV could be a NMOS transistor, acting as a current driver providing current with a proper magnitude to light-emitting device LT. Light-emitting device LT could be one or plurals of light-emitting diodes connected in series or in parallel. Dimming controller 10 provides driving signal S_DRVto the control gate of power transistor MNDRV. The current flowing through light-emitting device LT is monitored by dimming controller 10, as it is sensed by current-sense resistor RCS to provide current-sense signal V_CSat current-sense node CS. Dimming controller 10 receives dimming signal S_DIMfrom input node DIM to provide driving signal S_DRVaccordingly.
As shown in FIG. 1, the configuration of dimming controller 10 enables three different kinds of external connection to perform dimming control. For the first one, external circuit (not shown) generates and provides DC voltage V_DCused as dimming signal S_DIMto input node DIM, and the voltage level of DC voltage V_DCrepresents the luminance of light-emitting device LT. For the second one, variable resistor RDIM connects between input node DIM and ground voltage GND, and the resistance of variable resistor RDIM is converted by dimming controller 10 into DC voltage V_DCrepresenting the luminance of light-emitting device LT. How the resistance of variable resistor RDIM is converted into DC voltage V_DCat input node DIM will be detailed later on. For the third one, external circuit generates and provides PWM signal S_DIM-PWMused as dimming signal S_DIMto input node DIM, and the duty cycle of PWM signal S_DIM-PWMrepresents the luminance of light-emitting device LT.
In other words, dimming signal S_DIMcould be of DC or of PWM. Dimming signal S_DIMcould be categorized into one of two types: DC and PWM.
FIG. 2 demonstrates dimming controller 10 a, which could be dimming controller 10 in FIG. 1 according to embodiments of the invention. Dimming controller 10 a has type identifier 12, DC-to-PWM converter 16, multiplexer 17 a, LED driver 14 a, and constant current source 31.
DC-to-PWM converter 16 is a signal converter and, if dimming signal S_DIMis identified as DC, DC-to-PWM converter 16 converts dimming signal S_DIMinto PWM signal S_PWM. Shown in FIG. 2, DC-to-PWM converter 16 has signal generator 20, operational amplifier 24 and comparator 22. Please refer to FIG. 3, showing the correlation between dimming signal S_DIM, saw-wave signal S_SAWand PWM signal S_PWM. Configured as a unity-gain buffer, operational amplifier 24 reproduces the voltage level of dimming signal S_DIMat the non-inverting input of comparator 22. Signal generator 20 provides the inverting input of comparator 22 saw-wave signal S_SAW, which, like a clock, is periodically reset to its original starting voltage. Comparator 22 compares saw-wave signal S_SAWwith dimming signal S_DIMto generate PWM signal S_PWM. As shown in FIG. 3, PWM signal S_PWMis “0” in logic when saw-wave signal S_SAWexceeds dimming signal S_DIM, and “1” in logic when saw-wave signal S_SAWis lower than dimming signal S_DIM.
Type identifier 12 is connected to input node DIM, for identifying whether dimming signal S_DIMat input node DIM is of DC or of PWM, and accordingly provides selection signal S_SEL. Type identifier 12 in FIG. 2 makes selection S_DIM“1” in logic if it identifies dimming signal S_DIMas PWM, and “0” in logic if it identifies dimming signal S_DIMas DC.
According to embodiments of the invention, selection signal S_SELis determined in response to edges in dimming signal S_DIM. FIG. 4 exemplifies the waveform of dimming signal S_DIMthat has two falling edges FA1 and FA2, and a rising edge RA1. Type identifier 12 provides selection signal S_SELbased on whether there are an enough number of significant edges within a predetermined period of time. An edge is significant to be an edge of a PWM signal when its tilt is large enough. For example, if there are more than 4 rising or falling edges found within a window of 8 ms and each of these edges has a slope whose absolute value exceeds 0.1V/us, type identifier 12 identifies dimming signal S_DIMas PWM, making selection signal S_SEL“1” in logic. Two criteria must be satisfied to make selection signal S_SEL“1” in logic, for example. The first one is the count of rising or falling edges in a window of 8 ms must be larger than 4. The second one is each of these edges has a slope whose absolute value exceeds 0.1V/us. In the opposite, once type identifier 12 cannot find 4 edges, each having a tilt large enough, within a window of 8 ms for example, it identifies dimming signal S_DIMas DC, making selection signal S_SEL“0” in logic.
Taking the waveform in FIG. 4 for example, type identifier 12, according to an embodiment of the invention, deems falling edge FA1 starting when dimming signal S_DIMgoes down below reference voltage V_REF-Hand starts a window of delay time T_DELAY. At the end of delay time T_DELAY, type identifier 12 compares dimming signal S_DIMwith reference voltage V_REF-Lso as to know whether the absolute slope value of falling edge FA1 exceeds (V_REF-H−V_REF-L)/T_DELAYor not. Analogously, type identifier 12 deems rising edge RA1 starting when dimming signal S_DIMgoes up over reference voltage V_REF-Land starts another window of delay time T_DELAY. At the end of delay time T_DELAY, type identifier 12 compares dimming signal S_DIMwith reference voltage V_REF-Hso as to know whether the absolute slope value of rising edge RA1 exceeds (V_REF-H−V_REF-L) T_DELAY. In another embodiment of the invention, type identifier 12 checks whether or not the falling time for dimming signal S_DIMgoing down from reference voltage V_REF-Hto reference voltage V_REF-Lis longer than delay time T_DELAY, so as to know whether a falling edge is significant enough to be a falling edge of a PWM signal. The rising time for dimming signal S_DIMrising from reference voltage V_REF-Lto reference voltage V_REF-His also compared with delay time T_DELAYto know whether a rising edge could be deemed as a rising edge of a PWM signal. If there are an enough number of edges each having an absolute slope value larger than (V_REF-H−V_REF-L)/T_DELAY, then dimming signal S_DIMlooks like a PWM signal, and selection signal S_SELbecomes “1”. Otherwise, dimming signal S_DIMshould be a DC signal, and selection signal S_SELbecomes “0”.
Multiplexer 17 a in FIG. 2 has digital buffer 18 and multi-input, single-output switch 26. Digital buffer 18 is a signal buffer that reproduces and provides dimming signal S_DIMto multi-input, single-output switch 26 if dimming signal S_DIMis identified as PWM. Controlled by type identifier 12, multiplexer 17 a has two inputs receiving PWM signal S_PWMand dimming signal S_DIMrespectively. When type identifier 12 identifies dimming signal S_DIMas DC, multiplexer 17 a is controlled to select PWM signal S_PWMand forward it to LED driver 14 a, while isolating dimming signal S_DIMfrom LED driver 14 a. When type identifier 12 identifies dimming signal S_DIMas PWM, multiplexer 17 a isolates PWM signal S_PWMfrom LED driver 14 a, and digital buffer 18 passes dimming signal S_DIMon to multi-input, single-output switch 26, which, as controlled by selection signal S_SEL, forwards dimming signal S_DIMto LED driver 14 a. What multiplexer 17 a outputs to LED driver 14 a is always a PWM signal, which is either dimming signal S_DIMor PWM signal S_PWM, where PWM signal S_PWMrepresents dimming signal S_DIMwhen dimming signal S_DIMis of DC.
Selection signal S_SELshown in FIG. 2 controls multiplexer 17 a only, but the invention is not limited to however. According to embodiments of the invention, when dimming signal S_DIMis identified as PWM, DC-to-PWM conversion is unnecessary, so type identifier 12 sends selection signal S_SELto disenable or shut down DC-to-PWM converter 16, saving electric power. In the opposite, if dimming signal S_DIMis identified as DC, digital buffer 18 is optionally shut down or disenabled to save electric power.
LED driver 14 a receives a PWM signal only, and controls power transistor MNDRV to regulate current flowing through light-emitting device LT in response to what multiple-input, single-output switch 26 outputs. If the output of multiple-input, single-output switch 26 is “1” in logic, level shifter 28 outputs reference voltage V_REF, and operational amplifier 30 makes the current through light-emitting device LT about V_REF/R_CS, where R_CSis the resistance of current-sense resistor RCS. If the output of multiple-input, single-output switch 26 is “0” in logic, level shifter 28 outputs 0V, and operational amplifier 30 makes the current through light-emitting device LT about 0.
Constant current source 31 provides constant current I_SET, which, if there is variable resistor RDIM connected between input node DIM and ground voltage GND, goes through variable resistor RDIM to generate at input node DIM DC voltage V_DCused as dimming signal S_DIM. Accordingly, constant current I_SETconverts the resistance of variable resistor RDIM into DC voltage V_DC. While DC voltage V_DCor PWM signal S_DIM-PWMis directly supplied or defined from an external circuit with low output impedance, constant current I_SETcould not affect DC voltage V_DCor PWM signal S_DIM-PWMsince constant current I_SETis very small in magnitude.
FIG. 5 shows dimming methods 60 a in use of dimming controller 10 a in FIG. 2.
In step 62, dimming controller 10 a receives at input node DIM dimming signal S_DIM, which could be a PWM signal or a DC signal.
In step 64 following step 62, type identifier 12 identifies whether dimming signal S_DIMis of PWM or of DC to generate selection signal S_SEL, which controls multiplexer 17 a.
Step 68 a follows step 64 if dimming signal S_DIMis identified as DC. DC-to-PWM converter 16 converts dimming signal S_DIMinto PWM signal S_PWM.
Step 70 a, in response to selection signal S_SELgenerated in step 64, makes multiplexer 17 a select PWM signal S_PWMand forwards it to LED driver 14 a, which drives light-emitting device LT accordingly. Meanwhile, the signal path for dimming signal S_DIMfrom input node DIM, via digital buffer 18, and to LED driver 14 a is disconnected. In one embodiment of the invention, step 70 a disenables or shuts down digital buffer 18.
Step 72 a, in response to selection signal S_SELthat indicates dimming signal S_DIMas a PWM signal, makes multiplexer 17 select dimming signal S_DIMand forward it via digital buffer 18 and multiple-input, single-output switch 26 to LED driver 14 a driving light-emitting device LT. Meanwhile, multiplexer 17 a isolates PWM signal S_PWMfrom LED driver 14 a.
Dimming controller 10 a in FIG. 2 and dimming method 60 a in FIG. 5 have advantages as follows. If dimming signal S_DIMis of DC, PWM signal S_PWMrepresenting dimming signal S_DIMis generated for LED driver 14 a to drive light-emitting device LT. If dimming signal S_DIMis of PWM, dimming signal S_DIMis forwarded honestly to LED driver 14 a, which faithfully and quickly responds to turn ON or OFF light-emitting device LT. No matter dimming signal S_DIMis a PWM signal or a DC signal, dimming controller 10 a can always provide a proper PWM signal to LED driver 14 a to drive light-emitting device LT appropriately.
FIG. 6 demonstrates dimming controller 10 b, which could be dimming controller 10 in FIG. 1 according to embodiments of the invention. Dimming controller 10 b has type identifier 12, PWM-to-DC converter 19, multiplexer 17 b, LED driver 14 b, and constant current source 31. Several devices or circuits in FIG. 6 have been disclosed or taught by FIG. 2 and the relevant paragraphs, and their function and operation are not repeatedly detailed for brevity.
PWM-to-DC converter 19 is a signal converter and, if dimming signal S_DIMis of PWM, it is capable of converting dimming signal S_DIMinto DC signal S_DC. Shown in FIG. 6, PWM-to-DC converter 19 has digital buffer 18, resistor R1 and capacitor C1. Digital buffer 18 reproduces the logic value of dimming signal S_DIMand provides it to resistor R1. Resistor R1 and capacitor C1 together form a low-pass filter, capable of generating DC signal S_DCwhose voltage level represents the duty cycle of dimming signal S_DIM.
Multiplexer 17 b in FIG. 6, controlled by type identifier 12, has two inputs receiving DC signal S_DCand dimming signal S_DIMrespectively. Multiplexer 17 b has operational amplifier 24 and multiple-input, single-output switch 26. When type identifier 12 identifies dimming signal S_DIMas DC, operational amplifier 24, acting as a unity-gain buffer and a signal buffer, reproduces dimming signal S_DIMat its output and forwards dimming signal S_DIMto multiple-input, single-output switch 26, which continuously forwards dimming signal S_DIMto LED driver 14 b, but blocks DC signal S_DCfrom reaching LED driver 14 b. When type identifier 12 identifies dimming signal S_DIMas PWM, multi-input, single-output switch 26 in FIG. 6, as controlled by selection signal S_SEL, forwards DC signal S_DCto LED driver 14 b and blocks dimming signal S_DIMfrom reaching LED driver 14 b. What multiplexer 17 b outputs to LED driver 14 a is always a DC signal, which is either dimming signal S_DIMor DC signal S_DC, where DC signal S_DCrepresents dimming signal S_DIMif dimming signal S_DIMis of PWM.
LED driver 14 b receives a DC signal only, and controls power transistor MNDRV to regulate current flowing through light-emitting device LT in response to what multiple-input, single-output switch 26 outputs. If the output of multiple-input, single-output switch 26 has voltage level V_OUT operational amplifier 30 makes the current through light-emitting device LT about V_OUT/R_CS.
FIG. 7 shows dimming methods 60 b in use of dimming controller 10 b in FIG. 6. Some steps in FIG. 7 are the same or similar with steps in FIG. 5, so they are not repeatedly detailed here since they are comprehensible in view of related disclosure in the previous paragraphs.
Step 72 b, in response to selection signal S_SELthat indicates dimming signal S_DIMas a DC signal, makes multiplexer 17 b select dimming signal S_DIMand forward it via multiple-input, single-output switch 26 to LED driver 14 b driving light-emitting device LT. Meanwhile, selection signal S_SELcauses multiplexer 17 b to isolate DC signal S_DCfrom LED driver 14 b.
Step 68 b follows step 64 if dimming signal S_DIMis identified as PWM. PWM-to-DC converter 19 converts dimming signal S_DIMinto DC signal S_DC.
Step 70 b, in response to selection signal S_SELgenerated in step 64, follows step 68 b. Step 70 b makes multiplexer 17 b select DC signal S_DCand forward it to LED driver 14 b, which drives light-emitting device LT accordingly. Meanwhile, the signal path for dimming signal S_DIMfrom input node DIM, via operational amplifier 24, and to LED driver 14 b is interrupted.
Selection signal S_SELshown in FIG. 6 controls multiple-input, single-output switch 26 only, but the invention is not limited to however. According to embodiments of the invention, if dimming signal S_DIMis identified as PWM, operational amplifier 24 is optionally shut down or disenabled to save electric power. Similarly, when dimming signal S_DIMis identified as DC, type identifier 12 sends selection signal S_SELto disenable or shut down digital buffer 18, saving electric power.
Dimming controller 10 b in FIG. 6 and dimming method 60 b in FIG. 7 have advantages as follows. If dimming signal S_DIMis of DC, dimming signal S_DIMis forwarded honestly to LED driver 14 b, which faithfully and analogically adjusts the current through light-emitting device LT. The current through light-emitting device LT is V_OUT/R_CSif the voltage level of dimming signal S_DIMis V_OUT. While dimming signal S_DIMis identified as PWM, DC signal S_DC, representing the duty cycle of dimming signal S_DIM, is generated and forwarded to LED driver 14 b to drive light-emitting device LT. No matter dimming signal S_DIMis a PWM signal or a DC signal, dimming controller 10 b can always provide a proper DC signal to LED driver 14 b to drive light-emitting device LT appropriately.
While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

Claims

What is claimed is:

1. A dimming controller for dimming a light-emitting device, comprising:

an input node for receiving a dimming signal;

a type identifier connected to the input node, for identifying whether the dimming signal is of a first type or a second type;

a signal converter connected to the input node, for generating a first signal in response to the dimming signal, wherein the first signal is of the first type; and

a multiplexer controlled by the type identifier, having two inputs receiving the first signal and the dimming signal respectively, wherein the type identifier makes the multiplexer forward the first signal to a driver driving the light-emitting device if the dimming signal is identified as the second type, and the dimming signal to the drive if the dimming signal is identified as the first type.

2. The dimming controller as claimed in claim 1, wherein the first type is one of direct-current and pulse-width-modulation, and the second type is the other.

3. The dimming controller as claimed in claim 1, wherein the first type is pulse-width-modulation, the second type is direct-current, and the signal converter is a DC-to-PWM converter.

4. The dimming controller as claimed in claim 1, wherein in response to an edge of the dimming signal the type identifier provides a selection signal to the multiplexer.

5. The dimming controller as claimed in claim 4, wherein the dimming signal comprises edges in a predetermined period of time, and the type identifier provides the selection signal in response to whether each of the edges has a slope whose absolute value is larger than a predetermined value.

6. The dimming controller as claimed in claim 1, further comprising:

a constant current source for providing a constant current flowing through the input node.

7. The dimming controller as claimed in claim 1, wherein the type identifier disenables the signal converter if the dimming signal is identified as the first type.

8. The dimming controller as claimed in claim 1, wherein the multiplexer comprises:

a signal buffer; and

a multiple-input, single output switch controlled by the type identifier;

wherein the signal buffer reproduces the dimming signal and provides the dimming signal to the multiple-input, single output switch.

9. The dimming controller as claimed in claim 8, wherein the type identifier disenables the signal buffer if the dimming signal is identified as the second type.

10. A dimming method for a light-emitting device, comprising:

receiving a dimming signal;

identifying whether the dimming signal is of a first type or a second type to provide a selection signal;

converting the dimming signal to provide a first signal of the first type; and

forwarding one of the first signal and dimming signal in response to the selection signal to a driver driving the light-emitting device.

11. The dimming method as claimed in claim 10, comprising:

providing the selection signal in response to an edge of the dimming signal.

12. The dimming method as claimed in claim 11, wherein the dimming signal comprises edges in a predetermined period of time, and the dimming method comprises:

providing the selection signal in response to whether each of the edges has a slope whose absolute value is larger than a predetermined value.

13. The dimming method as claimed in claim 12, comprising:

providing the selection signal in response to whether a count of the edges is more than a certain number.

14. The dimming method as claimed in claim 10, wherein the first type is one of pulse-width-modulation and direct-current, and the second type is the other.

15. The dimming method as claimed in claim 10, wherein a signal converter converts the dimming signal to provide the first signal, and the dimming method comprises:

disenabling the signal converter if the selection signal indicates the dimming signal is of the first type.